how to query nested array mongodb

Question

I'm new to mongodb, and I'm trying to make a database that has posts and tags, where each post can have numerous tags, something like this (simplified)

posts: [
 post:{
   id: dkdkd,
   title: hello,
   body: hello world,
   tags: [1,2,3]
 },
 post:{
   id: ccc,
   title: hello2,
   body: hello world2,
   tags: [3,4,5]
 }
]

how can I get a list of all the tags? (i.e. returns [1,2,3,4,5])

Answer 1

You may use something like this to get all tags per document:

db.collection.aggregate([
{
  $project: {
    _id: 0,
    tags: {
      $reduce: {
        input: "$posts.post.tags",
        initialValue: [],
        in: {
          "$setUnion": [
            "$$value",
            "$$this"
          ]
        }
      }
    }
  }
}
])

Explained:

Project the array posts.post.tag and apply reduce operation with $setUnion between all documents tag array elements to receive distinct list of tags

playground

And something like this to take distinct tags per collection:

db.collection.aggregate([
{
$project: {
  _id: 0,
  tags: {
    $reduce: {
      input: "$posts.post.tags",
      initialValue: [],
      in: {
        "$setUnion": [
          "$$value",
          "$$this"
          ]
        }
      }
    }
  }
},
{
  $group: {
  _id: "Total",
  tags: {
    "$push": "$tags"
    }
  }
},
{
$project: {
  _id: 0,
  tags: {
    $reduce: {
      input: "$tags",
      initialValue: [],
      in: {
        "$setUnion": [
          "$$value",
          "$$this"
          ]
        }
      }
    }
  }
}
])

Explained: 1.project all tags per document. 2. Group all tags from collection. 3. setUnion to all tags arrays in collection.

playground2